In certain types of printing machines, the sheet that will receive the image is fed into the machine for each image printed. Different sheets are used depending on the imaging method used and the results desired. For high quality, photographic-like printing, the sheets typically consist of a base layer of a heavily milled paper or a polymeric paper-like compound. To achieve a high quality image, the surfaces of these sheets must have a high degree of smoothness, or gloss.
The image formed on the sheet may be created by either adding dyes onto the surface, removing dyes pre-deposited on the surface, or by activating dyes that are contained on the surface. In all these cases, coatings must be applied over the substrate material to receive the dye bearing material. Often, several coatings are applied to the surface to optimize the dye receiving and retention characteristics of the sheet. In thermal printing, the core may consist of a substrate, a smoothing polymeric overcoat, a whitening overcoat and a transparent dye-receiving coating.
The coatings each perform a function. The smoothing coat may be an extruded, soft polymer, such as polyethylene or polypropylene, to improve the smoothness of the core material. The bright white layer may consist of, for example, titanium dioxide, in solvent deposited over the smoothing layer. The reflective layer may contain a high concentration of, for example, titanium dioxide, to achieve light densities of less than 0.06 D. A final, transparent layer receives the dyes. This layer may be transparent so that light passes through the layer and dyes, reflects off the high brightness coating, and is retransmitted out to the observer.
In thermal systems, this transparent coating must also sustain the high temperatures that occur during printing. The surface coating is typically formed of a clear polycarbonate. In thermal systems, another type of sheet is also used. This sheet is substantially transparent to allow transmission of light through all of the layers. The base layer is typically a clear polymer, the high reflectivity layer is omitted, and additional coatings retain the dyes and optimize the dye transfer process.
The same techniques and methods are applicable to printing processes other than thermal printing. In the case of ink jet printing systems or electrophotographic systems, the smoothness, high brightness, and transparent overcoat layers are all needed to optimize the quality of the image. Formulation of the dye receiving, and other layers, may change from system to system to optimize the image quality. For example, in ink jet systems, the dye receiving layer should receive the ink solvents readily and dry quickly. In electrophotographic systems, the surface should be optimized to receive the electrostatically charged toner particles and to retain them until the fusing process occurs.
Certain processes allow for the prepositioning of dyes within the imaging surface. One technology is traditional silver halide imaging wherein a series of light activated particles are disposed on a multiplicity of coatings on the image bearing surface. After the printing of the image, wet or damp chemistry is used to selectively develop the image imprinted on the image surface. Another, later technology uses microencapsulated, diazo-based compounds that are responsive to heat or pressure for image development. In all of these systems, high quality images need to meet the general requirements outlined for high quality thermal printed images.
For all of these technologies, the coatings on the base layer are expensive, and, typically, in manufacturing printing media and image receiving media, the polymer coatings are applied to each side of the base material to optimize the printing properties. It is highly desirable to reduce costs by applying the coating to only one side of the sheet. This approach, however, requires correct loading of the sheet in the printer. The one-sided receiver media is less expensive to produce, but it requires an operator to load the machine or otherwise use the media with the sheet in the correct orientation. Labels, tags, or discontinuities in the edge surface can be employed to ensure correct loading and use of the media. Unfortunately, machine detectable discontinuities and labels are typically discernable by the user and therefore undesirable, and may reduce the aesthetics of the completed image. Accordingly, it will be appreciated that it would be highly desirable to identify the image receiving surface without affecting the aesthetics of the finished image.